1. Field of the Invention
The present invention relates to a transmission control system of an automatic transmission for a vehicle, which transmission control system carries out a downshift by changing engagement state of two friction elements.
2. Description of the Related Art
Conventionally, an automatic transmission combining a torque converter and a planet gear is widely used for a vehicle. The above automatic transmission makes the following control:                For carrying out a downshift (e.g., from 3rd speed to 2nd speed) in a coasting travel with an accelerator of the vehicle turned off (full closed), a 3rd speed friction element (namely, a disengagement side clutch which so far was engaged) is so controlled as to be disengaged, while a 2nd speed friction element (namely, an engagement side clutch) is so fedback gradually from a disengagement state to an engagement state. With this, the downshift to the 2nd speed is ended.        
Hereinabove, a driver of the vehicle stepping on the accelerator again in the feedback after an actual transmission start time (namely, after the engagement side clutch starts the engagement, or after torque conveyance is started) may cause a feedback for increasing an oil pressure of the disengagement side clutch so as to restrain a rapid increase in speed of an input shaft of the transmission (namely, a turbine speed).
In this case, however, a possible response delay in increasing the actual oil pressure relative to a duty ratio change of a solenoid may cause a rapid engine speed increase. This may make a determination that the oil pressure is short, thus a controller may increase the oil pressure more than necessary, resulting in a great transmission shock with both the disengagement side clutch engaged and the engagement side clutch engaged.
U.S. Pat. No. 5,445,579 (equivalent of Japanese Patent No. 3097339B2) discloses a control (referred to as “speed change control apparatus and method” in the above patent) solving the above inconvenience. More specifically, U.S. Pat. No. 5,445,579 (equivalent of Japanese Patent No. 3097339B2) discloses the control for preventing the above both-side engagements (disengagement side clutch and the engagement side clutch) by preventing an excessive oil pressure supply by setting up an upper limit of the oil pressure supplied to the disengagement side clutch. With the prevention of the above double engagements, the transmission shock can be restricted.
The control according to the U.S. Pat. No. 5,445,579 (equivalent of Japanese Patent No. 3097339B2) is, however, directed to the accelerator stepped on in the feedback after the actual transmission start time. In other words, the control according to the U.S. Pat. No. 5,445,579 (equivalent of Japanese Patent No. 3097339B2) is not effective for the accelerator stepped on before the actual transmission start time.
More specifically about this: Stepping on the accelerator before the actual transmission start time (a period from a time for commanding a transmission start to a time for starting torque conveyance by the engagement side clutch, during which period torque capacity of the disengagement side clutch is continuously decreased) may cause a slip to the disengagement side clutch, thus causing the rapid engine speed increase. Thereafter, the oil pressure may follow, thus rapidly engaging the engagement side clutch, resulting in the transmission shock.
Moreover about this: With the accelerator stepped on in the feedback after the actual transmission start time, the control according to the U.S. Pat. No. 5,445,579 (equivalent of Japanese Patent No. 3097339B2) restricts the transmission shock by merely setting up the upper limit of the oil pressure supplied to the disengagement side clutch. In this case, the rapid engine speed increase cannot be prevented assuredly. The driver therefore cannot obtain a preferred acceleration feeling.